More particularly, the present invention relates to a process of preparing stable suspensions of mineral fillers in acrylic and/or methacrylic monomers, suitable for bulk polymerization.
It is known that suspensions of mineral fillers in polymerizable liquids, having a high stability over the time necessary before being used in the preparation of finished articles by polymerization, can be obtained by using suitable suspending and viscosity-depressing agents.
In Italian patent No. 1,038,488, flowable, stable and hardenable suspensions based on a polymerizable organic liquid and on a particulated inorganic filler are disclosed, which are obtained in the presence of a copolymeric suspending agent (polymeric dispersant) containing in its chain a component solvatable by the hardenable liquid, and at least one group capable of associating with the mineral filler dispersed in the liquid basis.
To improve the mechanical properties of the finished articles, to the suspension is added a low-molecular-weight coupling agent, generally a silane, which is capable of forming strong chemical bonds with both the liquid basis and the mineral filler.
As an alternative, processes for preparing the above mentioned compositions have been disclosed, wherein as the viscosity depressants only coupling agents, possibly in admixture with surfactants of the anionic, cationic or non-ionic type, are used.
U.K. Patent Nos. 1,111,603; 1,132,261; and 1,140,542 disclose compositions based on alkyl-methacrylic type monomers, which contain high amounts of finely ground mineral fillers pretreated with a coupling agent selected from the organic derivatives of silicon.
According to the disclosures of these patents, a mineral filler, preferably on a siliceous basis, is treated, before being mixed with the polymerizable liquid monomer, with an organic derivative of silicon containing at least one functional group capable of reacting with the surface of the inorganic filler and at least one functional group capable of copolymerizing with the methacrylic monomer.
The functional group reactive with the mineral filler is preferably a hydrolysable group bonded to the silicon atom and which supposedly reacts with the hydroxy groups bonded to the surface of the inorganic material, to form very stable oxygen-bridge bonds.
The thus pre-treated inorganic material is added to the polymerized liquid basis and the obtained composition, to which surfactants are possibly to improve its flowability, results in a composition stable over time, pourable into moulds, and capable of yielding finished articles having good mechanical properties.
In European patent application No. EP-A-218,866 compositions based on esters of acrylic or methacrylic acid are disclosed, which contain a finely ground mineral filler, wherein the silanization of the above described inorganic material takes place in situ when the mineral filler is added to the polymerizable liquid basis.
According to the disclosure of that patent application, there are added to the polymerizable liquid basis an organic compound of silicon, also capable of polymerizing with the acrylic or methacrylic monomer and of reacting with the inorganic filler, and an alkylamine capable of acting as a catalyst for the above said silicon-organic compound.
However, to obtain a composition having good flowability, and therefore suitable for being poured into moulds, the mineral filler should be added to the liquid blend very slowly, in such a way that the viscosity of the obtained composition never exceeds 10 Pa.s.
In German patent application No. 3,531,913, the above mentioned drawback is overcome by adding to the composition an amount of up to 5% by weight of a suitable viscosity-depressing agent, such as soy lecithin.
We have now found that it is possible to prepare stable and flowable (fluid) suspensions of finely ground mineral fillers in a polymerizable liquid, by directly silanizing the inorganic material in situ, but without the drawbacks of the prior art (controlled addition of the mineral filler, or use of soy lecithin), by using particularly efficacious silanization catalysts.